3 points per tile enclosed. It's in every language in my original rulebook. And remember that you need to take up the highest spot around the lake with a worker in order to get the VPs. It's not a matter of putting down tiles and claiming VPs until kingdome come. You'll soon find that if you think it is, others will thank you for providing excellent scaffolding on which to erect valuable palaces. So don't even think about turning 'ignoring lake size' into a house rule: you'll unbalance the game.
Making lakes valuable was a very interesting design choice. That drives lake creation early in the game which is what forms the topography of the board for the rest of the game as lakes constrain later builds considerably. Play the rules as written.